Aromatic carboxylic acid herbicides such as aryl substituted carboxylic acid herbicides and aryloxy substituted acid herbicides have been used for many years. Many such herbicides are particularly useful in selectively controlling broad leaf vegetation. These aromatic carboxylic acids are generally converted to the ester form or the salt form to improve solubility in suitable solvents. The ester and salt forms of many of the herbicides are commercially available and sold either as a liquid concentrate to be diluted with water prior to use or as an aqueous solution. The ester forms are generally available as an organic solvent based emulsifiable concentrate. The salts, on the other hand are water soluble and available as an aqueous solution or solid.
Champion et al (U.S. Pat. No. 5,266,553) describes a method of manufacture of a dry water soluble salt composition which involves the use of a solvent comprising water as a major component. This process has a high energy consumption requirement and in order to minimise decomposition of the salt product sophisticated equipment is required to remove water at low temperature. Some salts, particularly 2,4-D dimethyl amine (DMA) salt, have been prepared using acetone as a solvent. In this process the 2,4-D is dissolved in acetone and the acetone solution is saturated with DMA. The 2,4-D DMA salt precipitates on formation and may be isolated by filtration and dried.
The formation of the salt in a solvent such as acetone also has significant problems which reduce the efficacy and economic viability of the process. While the salts are insoluble in acetone the presence of water significantly increases solubility so that as much as 10% of the product may remain dissolved.
Impurities are also formed in acetone. These include mesityl oxide and diacetone alcohol which must be purged from the system. These impurities also reduce the amount of solvent which can be reused. This significantly reduces product yield and produces a waste disposal problem. As a result isolation of useful materials from the waste stream is difficult and when the raw materials are relatively expensive, such as MCPP and Dicamba the process is uneconomical. Furthermore, some phenoxy acids such as MCPP can not be used because the resulting salts (e.g. MCPP/DMA) are too soluble in acetone.
There is a need for a process for preparing salts of substituted carboxylic acid herbicides in which the yield is high, the formation of impurities is minimised and the product can be easily isolated in solid form.